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Root Regeneration and Tolerance of Citrus Rootstocks to Root Rot Caused by Phytophthora nicotianae. J. H. Graham, Professor, University of Florida, IFAS, Citrus Research and Education Center, 700 Experiment Station Road, Lake Alfred 33850; Phytopathology 85:111-117. Accepted for publication 21 September 1994. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/Phyto-85-111.

Citrus rootstock cultivars varying in tolerance to fibrous root rot were evaluated for their ability to regenerate roots in the presence of potentially damaging populations of Phytophthora nicotianae. In chlamydospore-infested soils in the greenhouse, tolerance to root rot generally was exhibited as more rapid growth of undisturbed and pruned roots of the rootstocks trifoliate orange and Swingle citrumelo compared with Carrizo citrange, sour orange, Ridge Pineapple sweet orange, and Cleopatra mandarin. The capacity for regeneration of Volkamer lemon roots in the presence of P. nicotianae varied with experiments and was associated with differences in greenhouse temperature conditions. In a field trial with damaging populations of P. nicotianae, growth rates of regenerating roots of trifoliate orange and Swingle citrumelo were greater than for intolerant rootstocks, Carrizo citrange, sour orange, and Cleopatra mandarin. Regenerating roots of Volkamer lemon were infected and supported populations of P. nicotianae equivalent to intolerant rootstocks late in the season (October–December). Early in the season (April–June) Volkamer lemon roots apparently were tolerant because infection remained low and few propagules were detected until October. Young roots of all rootstocks supported higher levels of infection and rhizosphere populations of P. nicotianae than mixed-aged roots. In spite of comparable root infection, trifoliate orange had lower pathogen populations on regenerating roots and mixed-age populations of roots than intolerant rootstocks. Tolerance to root rot may be expressed as a greater capacity to regenerate roots under certain environmental conditions (e.g., Volkamer lemon) or the limitation of conversion of infection to propagules (e.g., trifoliate orange).